Glass joined to automobile bodies becomes an important structural part of the automobile as a whole. For instance, windshields must support the roof of the automobile in cases of crashes or rollovers. A current technique for joining glass to a painted auto body utilizes a urethane sealant for attachment of the glass surface to the painted surface. However, recent changes in the composition of automotive paints have made it difficult to satisfactorily adhere urethane sealant directly to painted surfaces.
In order to obtain acceptable levels of urethane adhesion to painted auto body surfaces, application of a priming top coat (i.e., a resin, a primer or tie coat) to the painted surface has become mandatory. Therefore, the current methodology for original equipment manufacture and for some cases of glass replacement requires a preliminary step of applying a priming top coat onto both the painted surface and the glass surface prior to joining the surfaces with the urethane sealant. Only through use of such a priming top coat can satisfactory adherence of urethane and joinder of paint-glass surfaces be achieved.
More particularly, in the case of windshield or backlite installation, the urethane sealant described above is applied to a portion of the auto body that is designed to receive the glass, commonly known as the pinchweld. In new auto bodies the pinchweld is coated with one or more layers of automotive paint, which is typically "high solids" enamel paint. When urethane sealant is applied to a painted pinchweld, the maximum strength of the adhesive joint between the painted surface and the glass surface will be the strength of the weakest member of the joint (either an interface or the sealant). It is desirable that the interface adhesion strength (i.e., paint-urethane or glass-urethane) be greater than the cohesive strength (within the sealant) of the adhesive itself. Therefore, when testing is performed on paint-urethane-glass joints, cohesive failure is preferable to adhesive failure.
In general, urethane sealants experience adhesive failure if applied directly to a painted pinchweld. Current practices for auto glass installation require cleaning of the painted surface prior to applying a primer top coat that promotes bonding of a urethane adhesive to the painted surface. Due to the diversity of paints used to coat auto bodies and the fact that paint composition affects the adhesive qualities of polyurethane, a variety of primer top coats must be employed for installation of glass components.
Thus, there is a need in the art for a method for treating painted and glass surfaces which provides improved adhesion of a urethane sealant to such surfaces without application of a primer top coat. The present invention provides a composition and a method effective for such treatment.